1. Technical Field
The present invention relates to speed-reduction/differential gear apparatuses for motor-driven electric vehicles, and in particular to one which has decreased loss in torque thereby having improved transmission efficiency leading to increased mileage per electric charge.
2. Description of the Related Art
A conventional speed-reduction/differential gear apparatus for electric vehicles is typically constituted by a combination of an electric motor, a planetary-gear speed reducer and a planetary-gear differential device. The speed reducer includes an input shaft which is integral with a motor shaft of the electric motor, and the differential device receives speed-reduced output from the speed reducer, as an input (Patent Literature 1).
In the differential device, an output is differentially distributed to two distribution members, i.e., the sun gear and the carrier. At the center of the sun gear, there is inserted and connected a first output shaft. The first output shaft coaxially penetrates the speed reducer's input shaft and the motor shaft which is integral with the reducer input shaft, connects to a motor-side constant-velocity joint, which is connected one of the driving wheels. On the other hand, the carrier is connected to a second output shaft. The second output shaft is connected to a differential-side constant-velocity joint, which is connected to another driving wheel.
In the above-described speed reducer and differential device, needle roller bearings are generally used for pinion gears which are included in the planetary gear mechanisms, as described in Patent Literature 1 and 2. An example is shown in FIG. 7.
FIG. 7 relates to a support structure for a pinion gear 1 in a speed reducer. A needle roller bearing 3 is placed between the pinion gear 1 and a pinion shaft 2. The pinion shaft 2 has its two ends supported by a carrier 4 and by a disc region 5 of a differential-side ring gear respectively. Thrust washers 6, 6 are placed between an end surface of the pinion gear 1 and the carrier 4, and between another end surface and a disc region 5. Lubrication to the needle roller bearing 3 is performed through an oil hole 7 which is made in the pinion shaft 2.